Role of Phosphorous, Nitrogen and Coldwork on Machinability of Carbon And Alloy Steels

April 26, 2011

Machinability of carbon and alloy steels is a shear process. Working the metal (Shearing to create chip) provides heat. The subsequent sliding of the produced chip on the face of the cutting tool provides heat as well.

Three ways to improve machinability include

  1. Optimizing the chemistry to provide for a minimum shear strength
  2. Adding internally contained lubricants
  3. Adjusting cold work

The steels that we are talking about are in large part composed of the ferrite phase. This is advantageous to us as machinists, because it has a relatively low shear strength.

Because ferrite is also ductile, it does not cut cleanly and tends to tear. Grade 1008 or 1010 are prime examples of  how pure ferrite machines. Long stringy, unbroken chips, torn surface finishes and lots of machine down time to clear “birds nests” are typical results.

Adding carbon up to a point improves machinability by adding a second harder phase (pearlite) into the ferrite. The good news is that up to a point, the chip formation is greatly improved, and surface finish improves somewhat. The bad news is that the shear strength of the steel is also increased. This requires more work to be done by the machine tool.

Addition of Nitrogen and Phosphorous can not only increase the shear strength of the ferrite, but also reduce the ductility (embrittle it).This ferrite embrittlement promotes the formation of short chips, very smooth surface finishes, and the ability to hold high dimensional accuracy on the part being produced. The downside is that these additions can make the parts prone to cracking if subsequebnt cold work operations are performed.

The graph below shows how cold work (cold drawing reduction) works in combination to reduce chip toughness, resulting in controlled chip length, improved surface finish, and improved dimensional accuracy of the part. To read the graphs, the Nitrogen content is shown in one of two ranges, and Phosphorous content is varied as is  the amount (%) cold work. You can see how the synergistic effects of these two chemical elements  when appropriately augmented by cold work, can drop the materials toughness  by as much as 80-90%.

 
 

Phosphorous and Nitrogen affect ductility; Cold work further activates their effect.

Add to that internal lubrication by a separate manganese sulfide phase or a lead addition, and now you can see how these factors can make grade 1215 or 12L14 machinable at speeds far, far, faster than their carbon equivalent 1008-1010. With greater uptime and tool life.

Internal Lubricant- Manganese Sulfides

And you thought that cold drawing just made the bar surface prettier and held closer in size…


6 Reasons Why Physics Is Critical To Contract Machining

June 2, 2010

Great question came in the other day.

“Since the computers control the machines, why do we need to have physics in our graduation curriculum?”

I won’t tell you the State Board of Education that was looking at removing Physics from the  high school curriculum.

Apparently they don’t see a need for a  person entering the Precision Machining workplace  to know any physics.

Who needs physics to push a button?

Who needs physics to push a button?

If they don’t understand the forces around them, how can they keep from getting hurt?

Here’s what I shared with them.

Since everything is computer controlled– that’s the new MAGIC, right?- why would any high school graduate going into the workplace these days need to know any physics?  I’m guessing that, “so they can understand how the electricity that powers his machine the computer, and the lights,”  isn’t a good enough answer.

1)Power and Work: All machines are horsepower rated. This determines what jobs they can perform. Materials are machined based on horsepower per cubic inch  of removal per minute.  By the State Board’s reasoning, “Since the clock takes care of the minutes, are we okay to just not know any of this?”

2) Mechanics: This is our craft! We need leverage, thread pitch, gear ratios, belts and pulleys. We calculate the surface feet per minute of rotating tools or workpieces,  given the RPM and diameter. Even the computer needs this info. Cams, clutches, springs, motors, friction and frictional losses- these are physics. Bearings,  force, stress, strain- these are applicable to understanding the machining task regardless of machine control type. Compressed air- expansion, horsepower required, volume, fluid flow…

3) Heat: Heat is the enemy in machining operations. Why not learn a little bit about this? Savvy shops today are using infrared thermography to detect bearing wear in equipment. Some kinds of tool failure are  caused by heat. Understanding insulation, conduction, thermal expansion and contraction are key if the parts will be in spec after they have cooled down  post machining.

4) Sound: Decibel measurement is important as applied to occupational exposure. Harmonics come into play on tools and workpieces as oscillation- chatter. Water hammer in plumbed systems and fluid power applications.

5) Light and optics: Non-contact gaging using lasers, optical projectors for quality control; optical flats for high precision measurements rely on counting interference bands…  We use portable spectrometers for product sorting.  Someone in the shop will need to have an understanding of spectrums, wavelengths, and emissions  if they are to be more than an idiot operated go/no  go gage.

6) Magnetism: Magnetism can cause surface finish problems if chips cling to work. There are several types of magnetic tests performed in our shops and those of our suppliers. They use eddy currents, permeability,  gauss, oersteds, saturation, coercivity. We employ  magnetism for proximity detection of parts, magnetic workholding , and for testing. It goes with out saying that it is magnetism in the electric motors that drives our machines.

What do you think about this topic? Do the people showing up looking for work have what it takes to understand your process? Or are they merely able to do what they are told?

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